1. Field of the Invention
The present invention relates to an image reading apparatus for reading the image of an original document by means of line image sensors such as CCD's, and more particularly to a color image reading apparatus for reading a color image with plural line image sensors of different colors arranged in the subscanning direction.
2. Related Background Art
In recent years there have been actively developed various color image reading apparatuses utilizing solid-state pickup components such as CCD's.
In such a color image reading apparatus, there is generally employed a line image sensor with different color filters arranged in the main scanning direction as disclosed by the assignee of the subject application in U.S. Pat. No. 4,891,690, or an arrangement of plural line image sensors of different color outputs in the sub scanning direction as disclosed by the assignee of the subject application in U.S. Pat. No. 4,558,357.
The former structure is advantageous in the simplicity of the structure, but is associated with drawbacks such that a stronger light source is required because the light-receiving area is limited, and such that a compensation is required for the aberration in the read image since the color filters are arranged along the main scanning direction.
On the other hand, the latter structure is associated with a drawback such that a complex circuitry involving memories is required for synchronizing different color signals in order to compensate the positional aberration among the plural line image sensors positioned along the sub scanning direction, but is free from the drawbacks inherent to the former structure.
The number of pixels in the line image sensor has been increasing in order to meet the requirement of image reading with a high image quality, and the former structure is disadvantageous in terms of device production, since each sensor requires a tripled number of pixels (in case of three-color reading) in comparison with the latter structure. Consequently, in order to meet the requirement of image reading with a high image quality, there is required a simple and inexpensive circuit for compensating the positional aberration, in the sub scanning direction, among the line image sensors of the latter structure.
For achieving an enlarged image reading, there is already known a method of reducing the relative moving speed between the line image sensors and the image in comparison with the case of equal-size image reading, thereby increasing the density of the image reading lines. When such enlarged image reading is applied to the above-mentioned latter structure, the number of lines between the neighboring line image sensors increases according to the rate of image enlargement.
Consequently the memory capacity, required for compensating the positional aberration of the line image sensors in the sub scanning direction, increases by the rate of image enlargement, in comparison with that required in the same size image reading. The decrease in said memory capacity is an essential condition for the above-mentioned simple and inexpensive circuit for correcting the positional aberration in the sub scanning direction.